IPtables firewalling

A Bare-Bones Guide to Firewalls:
http://linuxgazette.net//103/murray.html

Firewalling with netfilter/iptables:
http://linuxgazette.net//103/odonovan.html

Some howto's here:
http://www.linuxkungfu.org/

Gentoo:
http://forums.gentoo.org/viewtopic.php?t=159133&highlight=iptables
http://forums.gentoo.org/viewtopic.php?t=159710&highlight=iptables
http://www.gentoo.org/doc/en/gentoo-security.xml#doc_chap13

Oskar Pearson's tutorial:
http://iptables-tutorial.frozentux.net/

Iptables firewall generator:
http://linux.tkdack.com/module.php?mod=firewall